Daschle, for instance, supported the energy bill — which failed in the Senate and was opposed by a number of Democrats — and was the champion of its provision to expand the use of corn-based ethanol, an issue extremely important to his state. In a recently published autobiography, "Like No Other Time," the senator said that South Dakotans cast their ballots based on personal relationships, not party alliances.

"This is one of the reasons, I think, that I've never faced much hostility when I go home, even in the staunchest Republican strongholds," he wrote.

Last year, Daschle opted not to run for the Democratic presidential nomination, a decision he called excruciating. He has already raised $3.7 million for his re-election race, and his aides say he plans to raise $10 million. Dan Pfeiffer, a spokesman for the senator, said Daschle planned to be "the best organized, best-funded, best-prepared candidate in the country."

But Republicans, sensing Daschle is vulnerable, have been recruiting Thune for months. In November, Sen. George Allen, R-Va., chairman of the National Republican Senatorial Committee, visited South Dakota and publicly urged the former congressman to run.

Yet after the bruising campaign of two years ago, Thune took his time in making a decision.

Word of Thune's candidacy has generated excitement among Republicans in South Dakota, where the political landscape has been in turmoil since Rep. Bill Janklow, the state's only House member and a Republican, resigned after being convicted of manslaughter in a traffic accident. Thune recently announced he would not run for Janklow's seat, spurring speculation that he would take on Daschle.